1. Field of the Invention
The present invention relates to a means and method for detecting impurity concentration in a flowing gas stream, thereby to determine the end of a useful life of a gas purifier. The detection method can also be used to monitor gas lines for back-diffusion of an impurity species into the gas supply system.
2. Description of the Related Art
The rapid expansion of vapor-phase processing techniques, e.g., chemical vapor deposition, in the semiconductor industry has been associated with the deployment and use of manufacturing equipment that is totally reliant on the delivery of ultra-high purity process gases at the point of use in the semiconductor manufacturing facility. Currently, over 5 billion dollars worth of such equipment is in use.
Considering the impurities which are present in gas streams involved in semiconductor manufacturing, it is to be noted that the growth of high quality thin film electronic and opto-electronic cells by chemical vapor deposition or other vapor-based techniques is inhibited by a variety of low-level process impurities. These impurities can cause defects that reduce yields by increasing the number of rejects, which can be very expensive. These impurities may be particulate or chemical contaminants. Particulates are typically filtered out of the gas stream using extremely efficient commercially available particle filters, with particle filtration generally being employed at the point of use.
Chemical impurities may originate in the production of the source gas itself, as well as in its subsequent packaging, shipment, storage, and handling. Although source gas manufacturers typically provide analyses of source gas materials delivered to the semiconductor manufacturing facility, the purity of such gases may change because of leakage into or outgassing of the containers, e.g., gas cylinders, in which the gases are packaged. Impurity contamination may also result from improper gas cylinder changes, leaks into downstream processing equipment, or outgassing of such downstream equipment.
Chemical impurities that are of special concern in semiconductor manufacturing processes include water, oxygen, other oxidant species, and Lewis acids such as aluminum, boron or zinc-containing species. For some processes, impurities that may form active doping species in the resultant film are of concern, such as phosphorus or arsenic species in silicon processes. In general, the key chemical impurities must be held at levels of a few parts per billion or lower.
In support of the requirement for high purity process gases, a number of types of gas purifiers have been introduced that remove chemical contaminants from the semiconductor process gases at the point of use. These gas purifiers employ a variety of sorption processes to remove impurities, including physisorption processes, e.g. gas adsorption by zeolites or activated carbon, or various chemisorption processes, where the impurities adsorb to and chemically react with a component or components of the purifier.
Particularly useful in-line purifiers are based on sorption processes, wherein the impurity species are adsorbed and chemically reacted with scavengers bound to or incorporated in porous inert support materials. Such purifiers are described in U.S. Pat. Nos. 4,603,148, 4,604,270, 4,659,552, 4,761,395, 4,853,148, 4,797,227, 4,781,900, 4,800,189, and 4,950,419. This class of purifiers is quite versatile, since the immobilized scavenger may be varied and tailored to react with a large number of different impurities. Because the support material is usually porous, contact of the scavenger with the gas stream is extensive. Such gas purifiers are used to remove impurity species such as water, oxygen, other oxidant species, and Lewis acids, which have deleterious effects on the semiconductor manufacturing process. By varying the chemical identity of the scavenger, they may also be used to remove undesirable dopant species from the gas stream. For example, in silicon processing, arsenic and phosphorus are very active dopants, and thus traces of arsine and phosphine must be removed from silane, which may be the silicon source gas. European Patent Application EP 299,488 describes a metalated macroreticulate polymer having pendant functional groups used to remove arsine and phosphine impurities from silane gas.
Purifiers based on other sorption principles such as metal eutectic alloy getters are also employed. For example, European Patent Application EP 470,936 describes removal of impurities from hydride gases by passing the hydride gas over a hydrogenated getter metal in a chamber. In particular, disiloxane may be removed from silane using hydrogenated Zr-V-Fe getter alloy. Gases which may be purified in this fashion include SiH.sub.4, GeH.sub.4, NH.sub.3, AsH.sub..sub.3, SbH.sub.3 and PH.sub.3, all of which are used in semiconductor manufacturing. European Patent Application EP 365,490 describes a method for removing impurity gases from inert gases such as argon or nitrogen using a first sorbent of either a non-evaporable getter alloy of Zr-V-Fe or Zr-Fe and a second sorbent of a non-evaporable getter alloy of 5-30% Al, balance Zr. Both sorbents are pellets formed from alloy powder of average particle size below 125 microns, with the first sorbent being located at the gas inlet and the second at the gas outlet.
While the gas purifiers of the types described above are very effective at removing impurities from the process gas streams to very low levels, as they become saturated, they lose their ability to remove further impurities. The operator of the semiconductor manufacturing process needs to be able to determine when the gas purifier is no longer able to provide the level of purification efficiency required. The point at which the purifier is exhausted and needs to be exchanged for a new unit is referred to as the purifier's endpoint.
If the endpoint of the purifier is not detected and the purifier is not promptly replaced, the result can be that a large number of wafers undergo vapor-phase processing before it is recognized that compositional changes in the process gas stream flowed to the reactor are leading to high rates of rejection. Such high rates of rejection in turn significantly lower the efficiency and productivity of the semiconductor manufacturing plant, and generate substantial losses of potential product. The resulting off-spec microcircuitry articles thus constitute scrap which must be reworked, if this is even feasible, or else discarded as waste.
Adsorption-type purifiers can also be used in the back-diffusion scrubbing mode, whereby the purifier serves as an impurity scrubber that protects the gas supply against contamination caused by diffusion of one or more foreign components back into the supply lines. Back-diffusion can occur when mechanical components such as check valves and shut-off valves fail. Additionally, in low flow conditions, impurities can successfully diffuse against the convective forward flow. An example of a situation where back-diffusion is of concern is the case where an inert gas such as nitrogen is used to pressurize vessels containing liquids used in semiconductor manufacturing processes. Such liquids include sulfuric acid, isopropanol, acetone and the like, which can cause corrosion and contamination of the nitrogen supply system by back-diffusion under low flow conditions.
When the purifier is used for a back-diffusion scrubber, endpoint detection is critical. Back-diffusion is not planned for, and therefore it is impossible to predictively calculate the purifier's lifetime on the basis of flowrates, expected impurity concentrations, and so forth. Endpoint detection allows the immediate detection of a serious back-diffusion event, and the appropriate precautions to protect the gas supply may be mobilized. Use of two endpoint detectors disposed at separate points in the gas purifier's scavenger bed allows back-diffusion to be distinguished from normal exhaustion of the purifier. If the downstream endpoint detector signals purifier depletion before the upstream one does, back-diffusion can be diagnosed in a straightforward and simple way.
Accordingly, there is a pressing need in the semiconductor manufacturing industry to provide commercially viable systems for continuously monitoring the performance of gas purifiers to detect the purifier endpoint. Such endpoint detectors should preferably provide a signal that can be used, upon exhaustion of the purifier, to not only alert the operator that the purifier must be replaced, but also trigger steps, such as gas input valve closing or diversion of the input gas flow through a back-up purifier, to protect the semiconductor manufacturing process from contamination.
Such an endpoint detection method should not require the use of extremely specialized equipment or highly qualified personnel and should not be subject to effects due to unknown variations in impurity levels of the gas to be purified. The detection means should show a large and rapid response to the presence of the undesirable impurities in the gas stream, with reaction occurring in a time period that is short by comparison with the rate of movement of the impurity-containing front through the gas purifier.
The response should be easily converted to an electrical signal which can be used to, for example, close and/or open a relay and thereby trigger closing of a gas supply valve or diversion of the gas stream. The detector would desirably be sensitive to broad classes of impurities, and therefore be widely useful in various semiconductor manufacturing processes. In addition, the detector would preferably be inexpensive and constructed of materials that are compatible with semiconductor processing.
In an effort to provide an endpoint detector for gas purifiers used in semiconductor manufacturing processes, European Patent Application No. EP EP 438,036 describes a system for determination of the endpoint of an inert gas purifier containing gas sorbing material by measuring the inert gas pressure at the purifier inlet and outlet. An electronic measuring device records the difference between the measured values for electronic comparison with a predetermined value and a signal is given when the difference exceeds this value to indicate the end of the purifier's useful life. The gas sorbing materials are, for example, Zr, V and Fe-containing alloys for purification of He, Ne, Ar, Kr, and Xe. Unfortunately this method, based on differences in pressure, requires the use of costly pressure measuring instruments and electronic circuitry, whose use can only be justified for large scale gas purification plants. When smaller scale gas purification units such as in-line purifiers are used, whose throughput of gas is smaller, on the order of about 10 liters per minute, it is necessary to have a reliable indication of the approach of the endpoint, but also a reduced cost. Additionally, not all gas purifiers develop an increased pressure drop as they approach endpoint. This phenomenon is limited to purifiers of the metal alloy getter type.
To the same end, European Patent Application No. EP 449,791 describes a method and apparatus for determining the end of the useful life of a gas purifier which comprises (a) a gas purifier having an impure gas inlet in fluid communication with a housing containing a gas sorbing material; (b) the housing being in fluid communication with a purified gas outlet; characterized by (c) measuring the electrical resistance (Rx) between a predetermined point within the gas sorbing material and the housing; (d) comparing (Rx) with a predetermined resistance value (Rp); and (e) indicating when Rx is greater than or equal to Rp thus indicating that the gas purifier has reached its end of useful life. This method is only useful when the gas sorbing material is a material with high conductivity, such as a metal alloy getter, and when the conductivity of the gas sorbing material is significantly changed by the process of impurity scrubbing.
The method of European Patent Application No. EP 449,791 is not broadly applicable to the adsorption type purifier exemplified by U.S. Pat. No. 4,761,395. In this adsorption type of purifier, the gas sorbing material comprises a scavenger immobilized on an inert support material such as polystyrenedivinylbenzene copolymer or an activated alumina. Such a gas sorbing material has a very low conductivity when it is new, and as the purifier is exhausted, the conductivity does not change significantly. This adsorption type of purifier is widely used, since unlike the metal alloy getter purifier, it may be used at ambient temperature rather than needing to be heated to high temperatures. It is also more versatile, since the immobilized scavenger may be tailored to a large number of impurities, whereas the metal alloy getter purifier is limited to removal of those species that react with the heated metal alloy.
Other methods have been proposed which are based on changes in physical properties of the material which sorbs the impurity gas, such as, for example, a change in color. Such a system is presented in U.S. Pat. No. 4,782,226, which describes a method for determining the exhaustion of adsorption type purifier resin beads. A tube extends inside the purifier container having a transparent bulb sealed about its end. A fiber optic probe containing a transmitting and a receiving cable is positioned inside the tube such that the ends of the cables are proximate to the inner surface of the bulb. The cables are connected to a transmitter and receiver. A light beam is transmitted through the bulb, and is then reflected by the purifier resin beads. The receiving cable receives the reflected light and returns it to the transmitter and receiver. The transmitter and receiver compares the intensity of the reflected light received with the intensity of the original transmitted light. The fully reacted resin beads have a different reflectance value than the fresh beads, so when the reaction has taken place next to the bulb, the transmitter and receiver will sense the change. Upon sensing a change, the transmitter and receiver will activate a warning device, such as a light, which informs the operator that the reaction has been completed and the purifier needs replacing. This system has the disadvantage that not all gas purifiers undergo a color change upon exhaustion. For example, back-diffusion of arsine into many resin-based purifiers, e.g. the type described in U.S. Pat. No. 4,603,148, causes an insuffient color change. In addition, the fiber optics detector and its associated electronics are fairly complex.
In the context of other industrial processes, such as internal combustion engines, a variety of means and methods have been developed for in situ oxygen and moisture sensing. Most methods provide a response in the form of an electrical signal. The detection element is usually a gas-sensing metal oxide or semiconductor. For the purifier endpoint application, these methods suffer from several deficiencies. Many require elevated temperatures, such as tin oxide oxygen sensors. Because in these other applications, the sensor is expected to respond reversibly, the size of the signal and speed of the response are not sufficient for the endpoint application. In applications where the sensor is expected to be reversible, physically robust and have a long useful life, these constaints dictate against the type of high sensitivity detector of the present invention.
The presence of even small concentrations of impurity species in the process gas streams employed in semiconductor manufacturing is potentially deleterious. Even small levels of impurities on the order of parts per million (ppm) can cause inconsistent electrical properties in semiconductor devices manufactured by deposition techniques using impurity-containing gas streams.
It therefore is an object of the present invention to provide a simple, rapid, sensitive, and versatile system for detecting the point when a gas purifier is no longer able to provide a high level of purification efficiency, such as is required to protect semiconductor manufacturing processes. It is a further object of the present invention to provide a usable signal that, upon endpoint detection, can be used to activate processes such as valve closing or gas stream diversion, that protect the integrity of the manufacturing process or gas supply system.